I had a seat like that on my JD 210 garden tractor. I tried everything, Fantastik, scrubbing bubbles, bleach, SOS pads, nothing worked. I tried Tilex® Mildew Root™ Penetrator and Remover because I read about it on a forum somewhere. It melted the stains right off without even scrubbing. I had been trying on and off for a year before that product worked for me. Make sure to get the exact product above as they also make a "mold and mildew remover" that I can't vouch for on those stains.